Macroscopical examination at postmortem of 64 patients who died of their first transmural myocardial infarction (32 with anterior infarctions, 28 with posterior and 4 with lateral), during the first 30 days after the onset of symptoms, has shown that in 59 cases (92.2 p. 100) there was a totally occlusive thrombosis in the coronary artery. In all cases these thromboses were sited on the major coronary arterial trunk to zone of muscle which was destroyed, and on top of an ulcerated atheromatous plaque. The age of the thrombosis and the infarction were identical. There was no relationship between the presence (59 cases) or the absence (5 cases) of coronary arterial thrombosis with the age, sex, survival time or extent of the infarction. These postmortem findings are strongly suggestive of a fairly constant cause and effect relationship between coronary arterial thrombosis and acute transmural myocardial infarction.